The present invention relates to an apparatus for mounting a RF antenna array to a non-planar surface. More particularly, the present invention relates to an apparatus for attaching a plurality of discrete antenna elements to a surface of curvature such that the antenna array is made to conform to the contour of a complex three-dimensional surface.
While the invention disclosed herein may be used in a wide variety of RF sensing applications in which discrete antenna elements are mounted in conformity to nonplanar mounting structures, the preferred embodiment is directed to the antenna elements of a RF sensing apparatus of an aircraft, sensor pod, missile or surface array having a substantially cylindrical or conical array configuration. The antenna array may be coupled to an Anti-Radiation Homing (ARH) subsystem, for example, and the RF sensing apparatus used to detect a target and track its position using signals received in the form of energy emitted by or reflected from the target. The RF sensing apparatus embodying the present invention includes a passive antenna array comprising a plurality of individual broadband antenna elements, each generating a voltage when excited by an electromagnetic waveform emanating from the target. The elements are connected to a broadband receiver where the signals are processed and the signal information passed to a guidance processing unit for performing various guidance functions. For example, the guidance processing unit may perform angle-of-arrival determinations in which the direction of the source of a signal within the array""s field of view is located using signal information derived from the voltages sensed by the elements of the array.
A conventional RF sensing apparatus employs a plurality of RF antenna elements mounted on a stationary device or moving surface such as the nose of an aircraft, missile, sensor pod or other airborne apparatus. In more recent missile applications, the antenna elements have been confined to a structure aft of the nose section, which may house additional sensors. The antenna elements may then be distributed in one or more ring-like configurations protectively concealed below the skin of the cylindrically or conically shaped RF sensing apparatus. A low profile antenna array made compliant to its mounting surface while preserving the overall aerodynamic configuration of the airborne vehicle, or the surface continuity of the mounting surface, is generally referred to as a conformal antenna.
Positioning forward-looking conformal antenna elements behind a fairing, radome or similar protective and electromagnetically compatible mounting structure creates a formidable set of problems. First, the individual antenna elements, distributed circumferentially around the body of the RF sensing apparatus, are substantially shielded from signals originating on the opposing side of the RF sensing apparatus. Where the emitter signal is obliquely incident on the vehicle, the vehicle body shields as many as half the individual elements composing the array. This can significantly impair the performance of a direction-finding system using an array of conformal antenna elements. The antenna elements not shielded must then be capable of acquiring a minimum number of signals to generate independent phase and/or amplitude measurements at sufficiently high signal-to-noise ratios to resolve angular ambiguities and measure the angles-of-arrival accurately. The problem is further complicated by the polarization diversity of the antenna elements in the case of a cylindrical distribution of elements introduced in the preferred embodiment below. To this end, it is desirable to efficiently arrange a large number of compact elements in a dense array configuration.
As a second problem, the nose section of the RF sensing apparatus obstructs the antenna elements aft of it from signals originating from the direction immediately in front of the nose. The conformal nature of the element therefore conflicts with the preference for an end-firing antenna array. The challenge is then to design an array having a large effective field of view that is sensitive to both off-axis signals originating from the broadside of the RF sensing apparatus, as well as signals propagating along the vehicle""s centerline axis. Maintaining this degree of sensitivity across the field of view is achieved in part by mounting the antenna elements as close as practically possible to the surface of the RF sensing apparatus.
Ideally, an antenna element of a RF sensing apparatus is of high gain and provides reliable and uniform electrical performance over a wide range of frequencies. There are many such broadband antennas, including the spiral, log-periodic and traveling wave antennas, but few can be made small enough to satisfy the particular criteria necessary for missile and compact sensor suite applications. The antenna elements must lend themselves to being mounted in non-planar configurations and in sufficient number and density to acquire the signals necessary for performing direction-finding without producing significant electrical coupling between adjacent antenna elements. At the same time, an antenna element for missile ARH subsystems and other rugged, portable applications more generally, must be designed to withstand a range of demanding environmental conditions including severe shock, vibration, humidity, pressure and temperature variations.
One example of a suitable conformal antenna element is the microstrip antenna manufactured with printed-circuit technology. A typical microstrip antenna comprises a metal radiator and a ground plane separated by a dielectric layer with a thickness on the order of a tenth of a wavelength. The microstrip is then fed by a transmission line feed. While these microstrip antennas are small in volume and afford great variation in the number of elements and the array configuration, the manner of mounting or conforming the microstrip antennas to non-planar surfaces poses a challenge.
One fabrication technique for applying a microstrip antenna to a substantially curved surface involves constructing an antenna assembly from a sheet of dielectric material, then deforming the assembly to conform to the curved surface. The method as described is unsuitable for complex, curved surfaces, particularly those subject to stressing environments, because the various layers of the microstrip are under differing levels of tension/compression and are thus predisposed to delamination.
In a second method described in U.S. Pat. No. 4,816,836 to Lalezari, the fabrication of the microstrip is achieved in a two-step process in which a thicker first layer of dielectric is made to adhere to the curved surface and a second thinner layer of dielectric including the antenna circuit is shaped and secured to the first layer. Not only can the antenna element suffer from delamination, but the resulting antenna element possesses a substantially curved forward profile that gives rise to unacceptably large variations in the polarization orientation across the face of the antenna. This antenna as well as the previously described antenna and method of construction are therefore less desirable for use in stressful airborne vehicle applications.
In addition to the manufacture and installation of the individual antenna elements, a challenge remained to develop a RF sensing apparatus that exhibits the geometric and electrical uniformity necessary for implementing high-quality direction-finding. One prior art method of constructing a ring-shaped conformal array for mounting within the confines of a recessed channel in a missile system involves a three-step process. In the first step, the antenna elements with electrical connectors are pre-assembled in the shape of a ring. In the second step, the antenna elements are embedded in a compliant material such as epoxy in the shape of a ring that is severed at one point in the circumference. In the third step, the ring is expanded and the entire epoxy-embedded array slipped into the recessed channel of the RF sensing apparatus and then mounted.
This prior art assembly presents two problems. First, the array, having been embedded in epoxy, prevents individual elements from being replaced or repaired. As a wasteful and expensive result, the entire array must be discarded if any one element fails or an electrical connection is open or otherwise substandard. Second, a geometric error is introduced in the assembly of the array that perturbs the electrical uniformity of the array. The inner diameter of the ring is only slightly smaller than the outer diameter of the portion of the RF sensing apparatus to which it is mounted. After installation, the resulting gap created between the ends of the ring that nearly, but not entirely, meet is subsequently filled. The gap creates an electrical discontinuity which, if not properly cured, may give rise to variations in the phase and magnitude of signal reception by the antenna elements in proximity to the heterogeneity; such variations making the array unsuitable for direction-finding applications.
Below is described a novel apparatus for promoting the manufacturability, reliability, testability and uniformity of a conformal array for mounting antenna elements to non-planar surfaces.
It is an object of the present invention to provide an efficient, cost-effective means of manufacturing and assembling a conformal antenna array comprising a plurality of individual antenna elements, the elements being used for either reception or transmission of signals.
It is another object of this invention to provide a modular design which affords an opportunity to (1) individually test antenna elements prior to their installation in an antenna array and (2) test the antenna array as a whole prior to permanently and inalterably seating the antenna elements of the array.
It is another object of the present invention to create a conformal antenna array comprising antenna elements and optional calibration elements with the geometric symmetry necessary to achieve electrical uniformity among a plurality of antenna elements for accurately performing direction-finding.
The conformal antenna array of the present application comprises a plurality of broadband, conformal antenna elements. The antenna elements described are for the reception of incoming signals for a ARH subsystem but may be used for both receiving and transmitting RF signals in any number of antennas arrays, radar systems or platforms including aircraft, missiles, sensor pods or surface arrays. The antenna elements may be arranged in a non-planar configuration including one or more ring-like structures in a plane transverse to the principal axis of a cylindrically or conically shaped RF sensing apparatus. As described in the preferred embodiment, the antenna elements sense incoming signals that are processed by a receiver and guidance processing unit (of a radar or continuous wave system). The antenna elements may alternatively be used to transmit signals generated by a signal generator in a ground-based or airborne radar system. The signals sensed by a receive antenna element or generated for a transmit antenna element are conveyed by means of an electrical connection that is detachably connected to the corresponding signal processing or transmitting hardware.
The antenna array may further include calibration antennas which, when stimulated, induce voltages in the receive elements; the voltages being conveyed by means of a switching network to a multi-channel receiver for processing the signals and extracting direction-finding information.
Each receive element is indirectly mounted to a RF sensing apparatus, such as a missile seeker section, by means of a xe2x80x9ccarrier structure.xe2x80x9d A carrier structure is a rigid and conductive platform that is in mechanical and electrical contact with an individual receive element by means of a substantially permanent bond. The carrier structure serves as both a support structure and a ground plane to the corresponding antenna element. An individual receive element, in cooperation with the attached carrier structure, may be individually evaluated prior to installation on the seeker section or other mounting surface. After installation on the seeker section or other mounting surface, the plurality of receive elements composing the array may be collectively tested and any substandard units may be repaired or replaced individually.
The seeker section in the preferred embodiment has a substantially cylindrical or conical channel, groove or indentation machined around its circumference in the plane transverse to the principal axis of the aircraft, missile or sensor pod. Each carrier structure has an inner surface that substantially conforms to the seeker section channel or groove where they mate. Each carrier structure, in combination with an individual receive element, is secured to the non-planar surface of the seeker housing in a manner that is rigid but necessarily removable.
The carrier structure has an upper surface comprising a forward surface and a rear surface. The upper surface is directed toward the exterior of the aircraft, missile, sensor pod or other sensor housing, and the forward surface is an impedance matching section corresponding to the end of the RF sensing apparatus in the primary direction of signal propagation. The rear surface is made to conform to the contours of the base of a receive element where the carrier structure and receive element are joined. The receive element contemplated in the preferred embodiment is a broadband, horn antenna having a substantially planar surface where it mates with the carrier structure. The carrier structure and receive element are bonded by means that is rigid, permanent and conductive.
In the preferred embodiment, where forward is the direction toward the main beam, the forward portion of the upper surface of the carrier structure possesses a substantially uniform curvature between the forward end of the receive element and the edge of the carrier structure where it mates with the mounting structure. This forward portion is for impedance matching, and is the shape of a ramp to provide a smooth, continuous surface for conducting electromagnetic waveforms originating from the forward direction of the aircraft, missile, sensor pod or other sensing apparatus that propagate toward the conformally mounted receive element.
The carrier structure is made of a conductive material, and optionally includes support for removably mountable calibration elements. In the preferred embodiment, the calibration elements are affixed to two adjacent carrier structures such that the calibration element is equidistant from the two receive elements, thereby coupling RF energy into the elements with substantially the same amplitude and phase delay. A uniform electrical potential is maintained between any given receive element and the two adjacent calibration units. The calibration units are made to mount directly to the carrier structures instead of the seeker section to promote the integrity of the electrical continuity between the calibration and receive elements of the entire antenna array.
The principal benefit of the present carrier structure invention is twofold. First, the permanency of the bond between the carrier structure and the receive elements promotes the manufacture and testing of the individual receive elements and the array as a whole. Second, the detachability of the connection between the carrier structure and the seeker housing permits substitution of any individual receive element where necessary.
Although the carrier structure increases the overall parts count of the antenna array, an appreciable savings in the cost of manufacture is realized by obviating the need to discard the entire array apparatus due to an individual faulty component.
While the carrier structure also consumes a portion of the scarce volume in missile applications, the benefits afforded by the mechanical and electrical reliability of the antenna array work to offset the volumetric cost created by the inclusion of the carrier structures.